Fractures of bone generally involve complex injuries to surrounding soft tissues as well, which when treated conservatively often leads to misalignment, delayed or non-union of the fracture and stiffness of adjacent joints. It is well understood that fractures of bone that are treated with anatomical reduction and stable internal fixation using bone plates and bone screws are highly successful. Anatomical reduction and stable internal fixation are typically achieved using ‘compression’ screws wherein the hole in the bone plate is configured to enable the bone screw to glide within the hole and translate the bone fragment against the opposing bone fragment to enable compression of the two bone fragments against each other. Such a nature of fixation is typically understood to provide good clinical outcomes in fractures of the shaft or diaphyseal region of bone or for the fixation of joint fractures.
It is also well understood that bone healing can also result from relative stability or so-called flexible osteosynthesis whereby good clinical outcomes are obtained through maintenance of correct length, axis and rotation of the fractured bone fragments, rather than precise anatomical reduction and absolute stability or so called rigid fixation. Such a method of bone fixation is typically embodied in comminuted fractures where the goal is to maintain length of the bone and keep the end portions of the bone or the joints in correct position without directly fixing or manipulating the fracture zone itself. Existing solutions to enable this goal exist in the form of plate and screw systems where the bone screws are locked in the holes of the bone plate. The locked bone screws and bone plate form a stable construct and the stiffness of this construct provides stability to the fractured segments of bone. Locking of the bone screws in the plate hole ensures angular as well as axial stability, eliminates the possibility of toggling of the screw in the plate hole and thereby greatly reduces the risk of loss of reduction of the fractured fragments of bone, postoperatively. It is well known to a person skilled in the art that while these locking screws provide excellent angular stability and resistance to torsional and shear forces, they do not have the capability of compressing bone fragments.
As the regions of bone where fractures occur as well as the nature of fractures vary greatly, many fractures are not easily amenable to only rigid osteosynthesis with compression fixation alone or flexible osteosynthesis with angular stability alone. These situations led to an increasing need felt for bone plate assemblies which would allow either compression fixation or angle stable fixation with the nature of fixation of individual bone screws in individual plate holes being an intraoperative decision made by the operative surgeon depending on the nature of the fracture and the type of fixation required.
Bone plate assemblies that combine screws that enable compression of fracture fragments and screws that lock into the plate hole in a position of angular stability, typically having threads on the screw head, have been the subject of several patents and publications and are well known to those skilled in the art.
U.S. Pat. No. 6,669,701 discloses a bone plate with at least one hole including a first portion defining a substantially circular outer periphery and a second portion defining an elongated outer periphery and, with a plurality of threads disposed on the first portion, wherein the threads extend over an angle of 200 to 270 degrees at the upper surface of the bone plate and 180 to 230 degrees at the lower surface of the bone plate, with the first portion of the hole having threads configured to engage the threads on the head of a bone screw and the second portion of the hole configured to cooperate with the head of a different bone screw to provide compression to the fracture.
The bone plate as disclosed in the '701 patent has only a partial thread in the hole of the bone plate which does not circumscribe the diameter of the plate hole completely. Hence, screws used through such bone plates may not be able to absolutely maintain the desired fixed angular relationship between the bone plates and the bone screws under physiologic loads. Specifically, the screws within the plate are only partially captured and thus only partially surrounded by threads. Under high stress and loading conditions, the partially threaded hole in the bone plate may distort and allow the fixed angular relationship between the screw and the bone plate to change, resulting in a loss of fixation or loss of established intraoperative plate orientation.
US application 2007/0162020 remedies this disadvantage of a partial thread by disclosing a bone plate with a hole including a frustoconical portion and a thread that makes a complete revolution around the hole, a fastener with a spherical head to engage the frustoconical portion of the hole with a bone engaging portion, such that fine adjustment of a fracture is possible and a second fastener with a threaded head to engage the thread of the hole.
Such types of bone plate assemblies typically enable the use of either, one type of bone screw, typically with a spherical head that cooperates with a corresponding portion of the hole in the bone plate, to enable fracture fragment compression (compression screws) and rigid osteosynthesis, or, alternately, a second type of screw, typically with threads in the head that threadably engage with corresponding threads in a hole of the bone plate (locking screw), to enable an angular stability between the locking screw and the bone plate.
It is well understood that bone screws used in bone plate assemblies to compress bone fragments to one another, also compress the bone plate itself to the surface of the bone to obtain stability of the assembly. It is further well known that the resulting compression of the undersurface of the plate onto the surface of the bone leads to a compromise of the blood supply to the bone leading to stress shielding of the bone and resultant increase in the risk of poor clinical outcomes.
Further, fractures in osteoporotic bones form a significant proportion of all types of fractures that occur. One of the important characteristics of osteoporotic bones is that the poor bone quality does not allow good holding of the bone screws in the bone itself. U.S. Pat. No. '701 describes the use of bone plates with threads configured to engage the threads on the head of a bone screw to provide angular and axial stability in metaphyseal fragments of osteoporotic bone and a plate traction screw to compress the fracture. While this application addresses the poor holding power of the osteoporotic bone in metaphyseal fragments through the use of a screw with threads on the head to provide axial and angular stability, it does not address or provide a solution for the poor holding power of the osteoporotic bone for the traction screw passing through the plate. The obvious disadvantage of the described application is that such a fixation does not address the risk of loss of holding of the plate traction screw on account of the osteoporotic nature of the bone. Such a loss of holding would result in a direct loss of stability of the fixation of the fracture that has been carried out which would lead to a poor clinical outcome.
Therefore the prior art search reveals an urgent and acute need for an improved system of bone plates and bone screws wherein bone screws that are used to compress the fracture through a hole in the plate must also be locked in a relationship of angular and axial stability in the same plate hole. Further there clearly exists a need for bone screws that are enabled to be used as compression as well as locking screws that can compress the fracture in cooperation with a suitably designed bone plate and also be locked in a fixed angular relationship with the bone plate thereby ensuring angular and axial stability, without the use of any additional connection or mechanism that would cause a cumbersome additional step and further add an additional interface that could result in weakening of the bone screw-bone plate assembly.